This invention relates to a serial-type analog-to-digital converter (A/D converter) composed by serially connecting a folding circuit.
A conventional folding circuit diagram is shown in FIG. 6, in which B.sub.01 denotes a noninverting amplifier circuit having an output connected to the base of a transistor Q.sub.01. Reference number B.sub.02 denotes an inverting amplifier circuit having an output connected to the base of a transistor Q.sub.02. The emitters of the transistors Q.sub.01, Q.sub.02 are commonly connected to a current source IE.sub.1, and are wired to an output terminal P.sub.4.
The operation of the thus composed conventional folding circuit is explained by referring to the diagram of FIG. 7. The output voltage of the noninverting amplifier circuit B.sub.01 is indicated by the broken line, and the output voltage of the inverting amplifier circuit B.sub.02 is represented by the single-dot chain line. These voltages respectively enter into the bases of the transistors Q.sub.01, Q.sub.02, but since the emitters are commonly connected, the voltage of the output is determined according to the higher one of the potentials applied to the bases of the transistors Q.sub.01, Q.sub.02, and a folding characteristic as indicated by the solid line in FIG. 7 is obtained.
In the emitter-coupled output circuit having the above-described constitution, however, the current is divided into two transistors near the folding point, and the current flowing in one transistor is reduced by one half. Accordingly, the potential difference between the base and emitter becomes small, and nonlinearity results. Furthermore, the output voltage fluctuates depending on the temperature.
On the other hand, the structure of a conventional serial-type A/D converter is shown in FIG. 12, in which the analog output of the folding circuit 1-1 is connected to the analog input of a folding circuit 1-2, and the analog output of this folding circuit 1-2 is connected to the analog input of a folding circuit 1-3. Thereafter, the folding circuits 1-3 to 1-5 are similarly connected in a serial manner.
In the thus composed conventional serial-type A/D converter, an analog input signal is applied to the analog input terminal of the folding circuit 1-1, and with respect to this input voltage, the analog output voltage and digital output are determined as shown in FIG. 9. This digital output corresponds to the highest bit of the gray code. The second folding circuit 1-2 and the subsequent ones operate similarly, and their digital outputs continue sequentially to the second bit, third bit and so forth from the highest position of the gray code.
In such a structure, however, the precision of the folding circuit is determined by the precision of the entire A/D converter, and it was difficult to fabricate a high-precision serial-type A/D converter monolithically.
When the analog input voltage of the conventional serial-type A/D converter shown in FIG. 12 is scanned by a ramp voltage, the analog output of the folding circuit 1-5 is folded 31 times as shown in FIG. 13. Since each one of the folding circuits 1-1 to 1-5 contains a gain error and an offset error, the maximal value and minimal value of the folding output are deviated from the standard values. The minimal value has a very small error because it is due to only the folding circuit 1-5 in the final stage, but the error of the maximal value is doubled in deviation as passing every stage of the folding circuit, the error attributable to the earlier folding circuit is greater.
The maximum output voltage is expanded in the error, while the minimum output voltage contains an error only due to the final stage. In the folding circuit, it is when the input voltage is near the middle of the input voltage range that the output voltage becomes the minimum. By contrast, it is when the input voltage is at the end point of the input voltage range that the output voltage reaches the maximum. If this end point voltage is deviated from the specified voltage (input voltage range), its deviation is doubled to become the end point deviation of the output voltage.
In the folding circuit shown in FIG. 1, noninverting amplification is carried out around the operational amplifier A.sub.01, and inverting amplification is conducted around the operational amplifier A.sub.02, and the greater system of these two output voltages becomes predominant. FIG. 10 shows input, output characteristics when there is an offset error Vf in the inverting amplification system. If there is an offset, the deviation of the analog output voltage range has the greatest effect on the entire system. This deviation of the output voltage range is sequentially doubled in the subsequent folding circuits, and the maximal value of the analog output voltage in the final stage is largely deviated. FIG. 11 shows a relation between an analog signal input and an analog output of a certain stage, in which the vicinity of the maximal value is magnified. The line expressing the track with circles denote to the input, output characteristic curve without deviation, and the triangle and square marks denote the input, output characteristics of which maximal values are deviated to the positive side and negative side, respectively. Deviation of the maximal value to the positive side results in a widening of the input voltage range corresponding to the digital output at the folding point. To the contrary, when the maximal value is deviated to the negative side, the input voltage range corresponding to the digital output at the folding point is narrowed, and when the deviation is particularly large, a defect is formed in the digital output. In the example shown in FIG. 11, digital values "32, 33" are not delivered. These deviations of maximal value mean differential nonlinearity errors of the A/D converter. When fabricating a serial-type A/D converter monolithically, it has been difficult to obtain a high accuracy because of the error in the folding circuits.